A digital communications channel can be subjected to both intentional jamming events and to the occurrence of deep fading events. Such events can occur randomly, or they can occur periodically. Typically, Forward Error Correction techniques, combined with interleaving, have been used to combat these types of channel impairments.
A particular problem is created when one attempts to transmit a signal through a channel that is periodically interrupted by some object, such as a rotating blade of a helicopter, as some number (e.g., tens) of contiguously transmitted symbols may experience the deep fade due to obstruction by the helicopter blade. In this case the conventional FEC combined with interleaving may be insufficient to ameliorate the adverse effects of the periodic deep fade, resulting in an unacceptable degradation in received signal quality. The same problems can arise when a transmitted signal is subjected to intentional or unintentional jamming.